1. Field of the Invention
The present invention relates to a method for coating monocrystalline or polycrystalline metal substrates with a ceramic heat-insulating layer of zirconia by electron-beam vapor deposition, and to a method for coating turbine blades made of Ni-based superalloys.
2. Description of the Related Art
In order to reduce thermal losses in turbines and especially in turbine blades with interior cooling and thus to increase the efficiency of turbines, the turbine components are provided with heat-insulating layers. The method of electron-beam vapor deposition of Y partially stabilized zirconia (YPSZ) which is performed at a substrate temperature of about 1000.degree. C. has proven particularly effective for this purpose.
However, the life of the heat-insulating layers on their substrates is limited by the occurrence of supercritical stresses between metal substrate and ceramic layer. Accordingly, all measures for prolonging the life of such layers must always be directed to keeping these stresses as long as possible in the subcritical range. The stresses are not necessarily highest during the operation of the turbine because the application of the heat-insulating layers is effected at comparable temperatures and under stress-free conditions. However, they grow higher in the course of turbine operation, especially through the oxidation of the metallic adhesive layer which is situated between the metal substrate and the ceramic layer. When the turbine is turned off, the heat-insulating layers and the underlying metal substrates (e.g., turbine blades) cool down with thermal contraction. When the temperature is decreased from 1000.degree. C. to room temperature, the contraction is about 1.6% for Ni alloys and only 1.08%, on average, for heat-insulating layers made of YPSZ. The expansion coefficient of YPSZ is known to be anisotropic [H. Schubert: Anisotropic thermal expansion coefficient of Y.sub.2 O.sub.3 -stabilized tetragonal zirconia, J. Am. Ceram. Soc. 69 (1986) 3, p. 270-271]; it is higher along the c axis than along the a axis.